Add frontend lint and style tooling

[Arcadi4]

Summary

• Add root Prettier tooling with Tailwind class sorting support.
• Add frontend lint, style, typecheck, and aggregate check scripts.
• Keep this PR config-only; source formatting and CI/Ruff rollout are intentionally excluded.

Verification

• pnpm frontend:typecheck
• pnpm --filter frontend exec prettier --version
• lsp_diagnostics prettier.config.mjs
• git diff --cached --check before each commit